The metadata for the META can be con trolled via sidecar files during RR-pairs recording (search the documentation for sidecar)
The META response cannot be controlled that way, the META response as you probably know comes from the first RR pair processed. So I would always make a copy of one of the transactions, and make sure that by changing its name it would come alphabetically first in the list of RR pairs. Then change the response payload to what I wanted the META response to look like. And then I would give one of the request arguments a non-existing value such that the specific transaction which would also be created from this "meta"-rr-pair would never be matched as a specific response. (if this argument with the non-existing value is supposed to be a magic string than you also need to replace the response-value with the same non-existing string).